Internal combustion engine lubricant



United States Patent C) 3 085,978 INTERNAL COMBUSTION ENGINE LUBRICANT Bill Mitacek and Glenn E. Holman, Bartlesvllle, Okla,

assignors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Feb. 25, 1960, Ser. No. 10,865 9 Claims. (Cl. 252-334) This invention relates to an internal combustion engine lubricant. In one aspect, it relates to a novel lubricant additive mixture. In another aspect it relates to an improved internal combustion engine lubricant containing said lubricant additive. In another aspect it relates to a fuel-lubricant blend. In another aspect it relates to an improved method of operating an internal combustion engine using said fuel-lubricant blend.

The addition of alkyl lead compounds, such as tetraethyllead and tetramethyllead, to motor fuel in order to increase the anti-knock characteristics of the motor fuel, is well known. Upon combustion of the leaded fuel in an internal combustion engine, the alkyl lead compound is burned with the subsequent deposition of inorganic lead compounds upon the various surfaces of the combustion chamber. These deposits have a deleterious effect on the performance of the engines, particularly those of the two-cycle internal combustion type, which are frequently employed to power boats and lawn mowers. The majority of the motor users do use leaded gasoline because ,of the relatively greater availability at boat docks, service stations, and the like; therefore, the operational problems accompanying the use of leaded gasoline are common.

The trend toward increasing horsepower in outboard boat motors and other two-cycle internal combustion engines has compounded the internal deposition problem, as well as the problems of lubrication. For these engines, the lubricant is commonly mixed with the motor fuel. The generally recommended ratio of lubricating oil to fuel mixtures commonly falls in the range of about 1 quart of lubricant to each 2 to 12 gallons of gasoline.

The principal difiiculty encountered with the operation of the two-cycle engines is spark plug fouling. Other difticulties include ring sticking and port plugging. In the past, the use of additive-containing oil to reduce ring sticking and carbon deposits has been discouraged, because of the alliance of some of the metallic additive with lead salts to form electrically conducting whiskers which bridge the spark plug gaps resulting in plug misfiring. Thus, presently available lubricants are less than satisfactory for good two-cycle engine performance.

We have discovered a novel additive blend for com pounding into the lubricant, which lubricant in two-cycle internal combustion engines is mixed directly into the motor fuel. This additive blend consists essentially of synergistic proportions of an alkaline earth metal salt of a petroleum sulfonic acid, and a polymer of isobutylene. On the basis of the results obtained in extensive testing of moderately high horsepower outboard motors, we have found that a substantial improvement in two-cycle engine performance results from using a lubricant containing the additive blend of this invention over a lubricant lacking any additive, or containing an additive consisting essentially of (1) an alkaline earth metal petroleum sulfonate, or (2) polyisobutylene, individually.

An object of this invention is to suppress sparkplug fouling in a two-cycle internal combustion engine.

Another object is to provide a method of minimizin ring sticking and port plugging in a two-cycle internal combustion engine.

Still another object is to provide a novel motor fuellubricant mixture.

Still another object is to provide an improved method of operating a two-cycle internal combustion engine.

3,085,978 Patented Apr. 16.,- 1963 A still further object is to provide novel additive compositions for lubricants, particularly suited for improved lubrication and performance of a two-cycle engine.

Other objects, advantages and features of this invention Will become apparent to those skilled in the art Without departing from the scope and spirit of this invention, and it should be understood that the latter is not necessarily limited to the aforementioned discussion.

Thus according to the invention there is provided a lubricant additive mixture, suitable for use in a liquid hydrocarbon fuel, comprising a blend in synergistic proportions of (1) a calcium salt of a petroleum sulfonic acid, and (2) a polymer of isobutylene.

Further according to the invention there is provided a lubricant comprising a lubricating oil having said addi tive mixture incorporated therein in an amount within the range of from 1 to 5, preferably 1 to 3, volume percent of said lubricant.

Still further according to the invention there is provided a fuel-lubricant mixture comprising a liquid hydrocarbon fuel having said novel lubricant incorporated therein in an amount within the range of 2 to 11, preferably 2 to 6, volume percent of said fuel-lubricant mixture.

Yet further according to the invention there is provided a method of operating a two-cycle internal combustion engine which comprises passing said fuel-lubricant mixture through the fuel supply system of said engine to a combustion zone of said engine, burning said fuel, and said lubricant containing a small but effective amount of the additive blend for improving engine performance.

As mentioned above, it was surprising and unexpected that a synergistic effect is obtained when said calcium salt of a petroleum sulfonic acid is used in combination with said isobutylene polymer in the additive blend of the invention. In the practice of the invention, suitable proportions for said calcium salt and said polymer in the additive blend are usually from 60 to volume percent of said calcium salt and from 40 to 10 volume percent of said polymer. A presently more preferred range is from 67, to 80 volume percent of said calcium salt and from 33 to 20 volume percent of said polymer.

Expressed in other terms, the volume ratio of said sulfonate to said polymer in said additive blend is preferably within the range of 1.5:1 to 9:1, more preferably within the range of 2:1 to 4:1.

Based on the lubricant, suitable amounts of said calcium salt and said polymer are usually from 0.6 to 4.5 volume percent of said calcium salt and from 0.1 to 2 volume percent of said polymer. As noted, however, the additive blend ranges from 1 to 5 volume percent of said lubricant.

The polymers of isobutylene which are most commonly employed in this invention are homopolymers having the structure l'i an C C Liquid and resinous products having a molecular weight in the range of about 1,000 to about 50,000 are useful. Products having a mean molecular weight between about 10,000 to 15,000 are commonly preferred. For these products the average value of X in the above formula is from about 180 to 270. Various processes have been pro posed by which these polymers can be prepared; and it is within the scope of this invention to use polymers prepared by any of these processes. In many cases, the polymers are employed as solutions so as to facilitate handling and mixing. The diluent is desirably a lubricating oil or neutral oil. For example, a paraffinic 10-stock or 20-stock oil is employed as diluent. Paratone N, which is a 20 percent solution of polyisobutylene in a solvent neutral oil (150 SUS at F.) marketed by spasms the Enj ay Company, Inc., is useful in the practice of this invention.

In addition to the polymers of isobutylene which are described above, it is within the scope of the invention to employ copolymers formed by copolymerizat-ion of isobutylene with a minor amount of 1 or more copolymerizable monoolefins. The comonomers which can be used to prepare said copolymers are olefins having to 2 to 8 carbon atoms and the most preferred are the branched chain olefins. Examples of comonomers include: ethylene, butene-l, butene-Z, l-hexene, l-octene, Z-methyl-l butene, 3-methyl-l-butene and 2-methyl-2-butene. Preferably the comonomers are employed in amounts not greater than about 20 percent by weight of total monomers, i.e., isobutylene is used in amounts at least 80 percent by weight of the total monomers.

One process by which the above described polymers and copolymers can be prepared involves polymerization of the monomers with aluminum chloride or preferably boron trifluoride as catalyst. The polymerization is affected at low temperatures, generally between minus 50 F. and plus 100 F. A suitable diluent such as naphtha, hexane, pentane, butane, propane and the like facilitates handling of the viscous product and permits control of the reaction. Generally the reaction mixture contains 0.1 to 2 weight percent of the catalyst. The reaction is rapid, and reaction periods on the order of l to 200 minutes are commonly employed. At the end of the desired reaction period, the catalyst is neutralized, such as by addition of sodium hydroxide, and removed. The light unreacted hydrocarbons are removed by distillation. The product may be further separated by vacuum distillation to obtain a fraction of the desired molecular weight range. Details relating to the above and other processes for preparing these polymers are described in many U.S. patents, e.g., US. 2,901,471; US. 2,856,393; U.S. 2,727,022; US. 2,085,524 and U.S. 2,327,705.

The oil-soluble alkaline earth metal sulfonates which are employed in this invention are preferably the calcium salts of petroleum sulfonic acid. Commonly, the sulfonic acids are prepared by sulfonation of crude or refined pe troleum fractions at a temperature of about 50 to about 200 F. for a total reaction period of 20 to 90 minutes, with a sulfonation agent, such as fuming sulfuric acid, and represent a mixture of compounds. The petroleum fractions which are sulfonated are preferably solvent refined so as to remove asphalt, and dewaxed. Propane fractionated, solvent extracted fractions of Mid-Continent oils having a viscosity within the range of 50 to 230 SUS at 210 F. have been used. Sulfonat-ing agents commonly employed include sulfuric acid, sulfur trioxide and chlorosulfonic acid. Sulfur trioxide in sulfur dioxide has been found to be a satisfactory sulfonating agent. The oilsoluble sulfonic acids are then converted to the calcium salts. The molecular weight of these salts is commonly in the range from about 440 to 4000.

Suitable processes for preparing calcium petroleum sulfonates are described in the literature, for example, see US. Patents 2,509,863; 2,514,733; 2,532,997; 2,559,439 and 2,680,716. Further details of a preferred method of preparing a typical petroleum sulfonate can be found in the copending application of W. B. Whitney, Serial No. 665,985, filed June 17, 1957. Calcium salts of petroleum sulfonic acids are available as commercial products, such as designated by Lubrizol 239, marketed by the Lnbrizol Company.

In preparing the lubricant of the invention, the abovedescribed additive blend of the invention is blended with a conventional motor oil stock, prefer-ably a paraihnic oil which can be prepared by solvent refining and dewaxing of crude oil fractions. Processes for obtaining such oil stocks have been extensively described in the literature, for example, see Petroleum Refiner, vol. 31, No. 9, pages 194 and 198, September, 1952 and US. Patent 2,590,490. Particularly suitable stocks are prepared from selected Mid-Continent crude oils.

4. The properties of some preferred paralfinic stocks are tabulated below.

SAE viscosity grade, SUS at 210 F. 10 20 50 250 Gravity, API at 60 F 32. G 29. 4 27 24. 5 COO flash, F 390 470 505 610 C00 fire point, F. 440 535 635 685 Pour point F +10 +10 +10 SUS at F 100 328 1, 495 4, 200 SUS at 210 F 39. 5 54. 5 1]. 209 Viscosity 1nd 103 102 98 07 Color, NPA 1 2 3. 5 6 Color, ASTM, max-.. 1% 2% 8 Carbon residue 0.01 0 02 0.4 0 0 The test methods for the data in the above tabulation are as follows:

ASTM Flash and fire points, COC D92 Flash point, PM D93 Viscosity D88 Viscosity index D567 Pour point D97 Carbon residue, Conradson D189 Gravity D287 Color D As is well known, blends of two or more of such stocks can be employed to give an oil stock having the desired viscosity range.

The resulting finished lubricant is then ready for admixture with any suitable liquid hydrocarbon fuel generally employed as a motor fuel in two-cycle engines. Typical properties for some finished lubricants formulated in accordance with this invention are given in the tabulation below:

Viscosity, SUS at 100 F 220-2,600 Viscosity, SUS at 210 F 50-100 Viscosity index 98120 Pour point, maximum, F +10 Gravity, API 27.530 Flash point, minimum, F 425 The lubricants of this invention can be admixed with any carburetted internal combustion engine fuel, and are of particular application to an engine fuel having a volatility such that the 50 percent distillation point falls below about 310 F. These fuels may be obtained from mineral oils, or gaseous hydrocarbons derived from any source and by any of the known commercial methods of manufacture, such as straight-run distillation, catalytic cracking, thermocracking, destructive hydrogenation, polymerization, alkylation and hydrogenation. The fuels may also contain commonly used fuel additives such as alkyl lead anti-detonants, for example, tetraethyllead, lead scavenging agents, dyes, gum inhibitors, oxidation inhibitors and the like. The gasoline used should be substantially free of elemental sulfur. It should be doctor sweet. Gem erally, the fuel can be any desired hydrocarbon or mixture of hydrocarbons having a boiling point within the gasoline boiling range, i.e., 70 to 420 F.

EXAMPLE 1 A paraffinic 20-stock oil was chosen as the base stock for the preparation of the lubricant of this invention. Lubrizol 239, a commercial calcium sulfonate, was selected as the source of one of the components of the novel additive blend. This calcium sulfonate has a dencity of 1.0 gram per cubic centimeter at '60" F. and a viscosity at 210 F. of 1'50 SUS. This product contains about 3.5 weight percent calcium and about 2.3 weight percent sulfur.

The engine was operated in specially designed water tanks under the following conditions:

The other additive blend component was Par-atone N, 5 iii? "ggs igz a polyisobutylene, furnished as a 20 percent solution in C oolant Out 146-156 F. a diluent oil, the diluent oil having a viscosity of 150 Tank Temperature LESS than 1000 F SUS at 100 F. Therefore, to make all the runs with the several lubricant compositions as nearly comparable as possible, since the volume of polymer solution used The engine Was Operated through a Cycle of minutes va ied fro o o iti t th a i il dil t at theabove full throttle conditions and then idled for il h i a i it f 150 SUS at 109 F was five minutes for a total period of 100 hours with each ployed, as required, to maintain the percentage of the fuel-lublrlcant uttereoil components in the finished lubricant constant for The 1011115911 engme p y has tWO Spark p A each run. spark plug failure, for the purposes of these tests, was in- The properties of typical samples of Paratone N are mated y either the complete pp of the engine, y as f ll rough idling, or by significant loss of engine speed. When a plug failure did occur, it was replaced with a new plug. Viscosity, SUS at 210 F. 3,000 Ta l I gives the operating conditions and results Flash point, Cleveland open cup, F. 400 which measure two-cycle engine performance employing Specific gravity at 60 F. 0:875 lubricants incorporating the additives of this invention.

TABLE I Candidate Oils and Results of Engine Tests Using P0lyz's0bulylene and Calcium .S 'ulfonaze Averages of hourly engine Exhaust Spark operating conditions Piston burning and Ring port plug Lubricant composition scutdng sticking, plugging, failures,

10=tree 100= number Water Water Inlet clean Rpm. inlet, outlet, air, F. F.

cofiposittion A: 90% 20 stock, 8% neutral, 2% Ca None 5.0 98 4 4, 825 73 156 70 ofimfiitian B: 92% 20 stock, 8% neutral No. 1 piston-hole burned 2.8 98 11 4,950 72 15s on top ring land. Composition 0: 90% 20 stock,8% neutral 2% polymer- None 4.0 98 11 4,952 66 149 61 Composition D: 90% 20 stock, 8% neutral, 1% polydo 10 98 5 4, 940 146 49 mar, 1% Ga sulfonate. Composition E: 90% 20 stock, 8% neutral? 0.5% poly- --.-.d0 7. 5 98 2 5,029 147 46 mer, 1.5% Ca sulfonate.

! Added as a 20% solution in a diluent oil as supplied under the trade name of Paratone N. i The calcium sulfonate was a commercial product designated as Lubrlzol 239.

3 The neutral oil had a Saybolt viscosity of 150 sec. at 100 F.

The calcium sulfonate and the polyisobutylene solution were mixed with the ZO-stock oil. Mixing was facilitated by warming to a temperature of about 120 F. The

volume ratio of sulfonate to polymer was varied in each lubricant compounded, but the total additive comprises a two percent of the final lubricant in every case, but that of the control run.

Also, the sulfona-te and polymer additive components,werecompounded separately with the paraflinic'base stocks. For a series of runs, fuellubricant mixtures, having lubricant without'any additive, the additive components in combination, and the additive components singly, were prepared.

iuel for 100 hours or operation of a two-cycle internal combustion engine.

A 1958, 35 horsepower Johnson Super outboard was chosen as the test engine for this investigation. This "engine is a modern relatively high-output model and is similar to models made by several other manufacturers and is recommended for lubricant oil evaluation purposes.

The run made with Composition A gives the elfect of using two percent of calcium sulfonate alone as the lubricant additive. A control run using oil Composition B, was madeto determine the effect, if any, of theneutral hydrocarbon oil alone, which acts as the solvent for the polymer. The results indicate that no beneficial effect could be attributed to it. The run made when using Composition C indicates the lack of benefit derived from employing the polymer alone as an additive. Composition D shows the improvement from using equal parts of the two additive components as compared to the results I achieved when employing the polymer or calcium sulfonate alone. A clear showing of synergism is noted when employing lubricant E, in that the number of spark plug failures, when employing a total of two percent of the two additive components, with the volume ratio of sulfonate to polymer being 3 to l, was 50 percent less than the next best results, obtained when using Composition A. The results are clearly better than was realized when using either additive component alone in the same amount.

EXAMPLE 2 Another series of runs were made employing lubricant oil wherein the total additive blend was not limited to a aosseva with the Mercury engine, 1 quart of lubricant was used for each 4 gallons of gasoline.

Table III gives results contrasting operation of the twocycle engine with the lubricant of this invention to a 5 lubricant with only olefin polymer as additive.

TABLE II Candidate Oils and Results of Engine Tests Using Polyisobutylene and Calcium Sulfonaze 100-hour engine test Exhaust Lubricant composition eosity Piston Piston Ring 0 t Spark plug index burning varnish sticking fi failures. Remarks and 10=c1ean 10=iree p number scuifing clean Cortnplgsition F: 77% stock, 23% 60 101 None.-. 5.1 6.5 95

s cc Composition G: 62.2% 20 stock, 35.8% 101 do 7.1 7.8 88 High level combustion chamber de- 50 stock. 2.0% Ca sulfonate. posits. Composition H: 79.8% 20 stock, 15.2% 50 110 do- 8.8 7. 0 98 stock, 0.0% polymer. 2.4% neutral.

2% Ca sulionate. Composition I: 93.0% 20 stock. 0.9% 115 do. 8.4 9.0 99

tpolyncr, 3.0% neutral. 1.9% Ca sulona 0. Composition J: 38.4% naphthenic oil. 5 in hours Chunks of deposits had broken loose 57.0% 20 stock. 0.5% polymer, 2.0% from piston crown.

neutral, 1.5% Ca sulionate.

3 The neutral oil had a Saybolt viscosity of 150 sec. at 100 F.

The results of this series of engine runs showed that none of the compositions gave piston burning and sending. Also, ring sticking and port plugging were satisfactory when the novel additive was used. The advantages for the fuel blended with the lubricant incorporating the synergistic combination of compounds of this invention are again especially evident by reference to the spark plug failures. The Composition F, containing no additive, when used as a lubricant, gave five plug failures in a 100 hour test. Use of the calcium salt alone, as in Composition G, gave only a slight improvement over Composition F. Compositions H and I show clearly the enhanced effect of using calcium sulfonate in conjunction with polyisobutylene as a two-cycle engine lubricant additive. Composition I is presented to show that substitution of part of the paraffnic oil stock by a naphthenic oil gave poor results. Thus, it is highly important that paraffinic oils be used as the base stock in the practice of this invention.

EXAMPLE III A third series of runs were made, but this time a horsepower Mercury outboard engine was employed to determine the effects of the lubricant of this invention when used with an unleaded gasoline in a high horsepower engine.

The paraflinic 20-stock oil, calcium sulfonate and isobutylene polymer employed were compounded together exactly as indicated in Example I. Two lubricants, K and L, were prepared. The first comprised namely: 93% of 20-stock oil, 5% Paratone N 1% polymer and 4% neutral oil), and 2% Lubrizol (calcium sulfonate). The second contained only 5% Paratone N additive to 95% 20-stock oil, that is, no calcium sulfonate was included.

Both runs were 100 hours in duration with engine being operated through cycles of minutes at full throttle and then idled for 5 minutes. The Mercury engine employs four spark plugs. A spark plug failure was determined here as was the case in Example I. For the runs 1 Trace scufliing No. 1 piston.

It should be noted the novel lubricant K provides exceptional engine cleanliness with respect to ring sticking, piston varnish, and port plugging, while the runs extended over the full 100 hours. This is clearly a definite improvement in performance over that resulting from using a lubricant lacking the additive of this invention. Thus, the novel lubricant is also beneficial where an unleaded motor fuel is available for a two-cycle engine.

While certain embodiments of the invention have been described for illustrative purposes, the invention obviously is not limited thereto. Various other modifications will be apparent to those skilled in the art in view of this disclosure. Such modifications are within the spirit and scope of the invention.

That which is claimed is:

1. A two-cycle, internal combustion engine lubricant consisting essentially of from 99 to volume percent of a parafiin base lubricating oil and from 1 to 5 volume percent of a lubricant additive blend consisting essentially of synergistic proportions of a polymer of isobutylene and a calcium salt of petroleum sulfonic acid suificient to reduce spark plug failures, wherein the volume ratio of said calcium salt to said polymer is in the range of 2:1 to 4: 1, and said polymer has a mean molecular weight in the range of from about 1,000 to 50,000.

2. The lubricant of claim 1 wherein said polymer is selected from the group consisting of polyisobutylene and a copolymer of isobutylene with at least one other monomer selected from the group consisting of ethylene, butene-l, butene-Z, l-hexene, l-octene, Z-methyl-l-bntene, 3-methyl-1-butene, and 2-methyl-2-butene.

3. A two-cycle internal combustion engine lubricant comprising about 97 volume percent of a paraflin base lubricating oil and about 3 volume percent of a lubricant additive blend, said lubricant blend consisting essentially of about 2 volume percent of a calcium salt of a petroleum sulfonic acid and about 1 volume percent of a polymer of isobutyene, said polymer having a mean molecular weight in the range of from about 1,000 to 50,000.

4. The lubricant of claim 3 wherein said polymer is polyisob-utylene.

5. A two-cycle internal combustion engine lubricant comprising about 98 volume percent of a paraflin base lubricating oil and about 2 volume percent of a lubricant additive blend, said blend consisting essentially of about 1.5 volume percent of a calcium salt of a petroleum sulfonic acid and 0.5 volume percent of a polymer of isobutylene, said polymer having a mean molecular weight in the range of from about 1,000 to 50,000.

6. The lubricant of claim 5 wherein said polymer is polyisobutylene.

7. A incl-lubricant mixture for a two-cycle internal combusion engine comprising from 89 to 98 volume percent of a liquid hydrocarbon fuel and from 11 to 2 volume per cent of a lubricant composition consisting essentially of from 99 to 95 volume percent of 'a paraflEin base lubricating oil and from 1 to 5 volume percent of a lubricant additive blend consisting essentially of synergistic proportions of a polymer of isobutylene and a calcium salt of petroleum sulfonic acid sufficient to reduce spark plug failures, wherein the volume ratio of said calcium salt to said polymer is in the range of 2:1 to 4:1, and said polymer has a mean molecular weight in the range of from about 1,000 to 50,000.

8. The fuel-lubricant mixture of claim 7 wherein said polymer is selected from the group consisting of polyisobutylene and a oopolymcr of isobutylene with at least one other monomer selected from the group consisting of ethylene, butene-l, butene-2, l-hexene, l-octene, 2- methyl-l-butene, 3-methyl-l-butene, and 2-methyl-2- butene.

9. In the method of operating a twocycle internal combustion engine wherein air and a hydrocarbon ruel are burned in a cylinder of said engine, and wherein said uiuel contains a lubricant for lubricating said engine, the step of introducing into said cylinder a fuel-lubricant mixture comprising from 89 to 98 volume percent of a liquid hydrocarbon fuel and from 11 to 2 volume percent of a lubricant consisting essentially of from 99 to volume percent of a parafiin base lubricating oil and from 1 to 5 volume percent of a lubricant additive blend consisting essentially of synergistic proportions of a polymer of isobutylene and a calcium salt of petroleum sulfonic acid sufiicient to reduce spark plug failures, wherein the volume ratio of said calcium salt to said polymer is in the range of 2:1 to 4:1, and said polymer has a mean molecular weight in the range of from about 1,000 to 50,000.

References Cited in the file of this patent UNITED STATES PATENTS 2,638,446 Wasson May 12, 1953 2,683,120 Jennings et al. July 6, 1954 2,849,398 Moody et al. Aug. 26, 1958 2,896,593 Riemenschneider July 28, 1959 3,015,320 Brennan et al. Jan. 2, 1962 FOREIGN PATENTS 219,994 Australia Feb. 5, 1959 773,999 Great Britain May 1, 1957 OTHER REFERENCES Scientific Lubrication, March 1959, pp. 12-30 pertinent. 

1. A TWO-CYCLE, INTERNAL COMBUSTION ENGINE LUBRICANT CONSISTING ESSENTIALLY OF FROM 99 TO 95 VOLUME PERCENT OF A PARAFFIN BASE LUBRICATING OIL AND FROM 1 TO 5 VOLUME PERCENT OF A LUBRICANT ADDITIVE BLEND CONSISTING ESSENTIALLY OF SYNERGISTIC PROPORTIONS OF A POLYMER OF ISOBUTYLENE AND A CALCIUM SALT OF PETROLEUM SULFONIC ACID SUFFICIENT TO REDUCE SPARK PLUG FAILURES, WHEREIN THE VOLUME RATIO OF SAID CALCIUM SALT TO SAID POLYMER IS IN THE RANGE OF 2:1 TO 4:1, AND SAID POLYMER HAS A MEAN MOLECULAR WEIGHT IN THE RANGE OF FROM ABOUT 1,000 TO 50,000. 